1. Field of the Invention
The present invention relates to a sensor system for determining at least one flow property of fluid medium.
2. Description of the Related Art
Numerous methods and devices are known from the related art for determining at least one flow property of fluid media, i.e., liquids and/or gases. The flow properties may be basically any given physically and/or chemically measurable properties which qualify or quantify a flow of the fluid medium. In particular, a flow rate and/or a mass flow and/or a volume flow may be involved.
The present invention is described below in particular with reference to so-called hot film air mass meters, as described, for example, in Konrad Reif (publisher): Sensoren im Kraftfahrzeug [Sensors in Motor Vehicles], Edition 1, 2010, pages 146-148. These types of hot film air mass meters are generally based on a sensor chip, in particular a silicon sensor chip, having a sensor diaphragm as a measuring surface or sensor area over which the flowing fluid medium is able to flow. The sensor chip generally includes at least one heating element and at least two temperature sensors which are situated, for example, on the measuring surface of the sensor chip. A mass flow and/or volume flow of the fluid medium may be deduced based on an asymmetry of the temperature profile detected by the temperature sensors, which is influenced by the flow of the fluid medium. Hot film air mass meters are usually designed as plug-in sensors which are permanently or replaceably introducible into a flow tube. For example, this flow tube may be an intake tract of an internal combustion engine.
A partial flow of the medium flows through at least one main channel provided in the hot film air mass meter. A bypass channel is provided between the inlet and the outlet of the main channel. In particular, the bypass channel is designed in such a way that it has a curved section for deflecting the partial flow of the medium which has entered through the inlet of the main channel, the further course of the curved section merging into a section in which the sensor chip is situated. The latter-mentioned section represents the actual measuring channel in which the sensor chip is situated. A means is provided in the bypass channel which conducts the flow and counteracts separation of the flow of the media partial flow from the channel walls of the measuring channel. Furthermore, in the region of its opening, which points against the main flow direction, the inlet area of the main channel is provided with angled or curved surfaces which are designed in such a way that medium flowing into the inlet area is diverted from the portion of the main channel leading to the sensor chip. As a result, liquid or solid particles contained in the medium may not reach the sensor chip due to their mass inertia, and may soil the sensor chip.
In practice, these types of hot film air mass meters must meet numerous requirements and constraints. In addition to the aim of reducing an overall pressure drop at the hot film air mass meter with the aid of suitable flow designs, one of the main challenges is to further improve the signal quality as well as the robustness of such devices with respect to contamination by oil and water droplets, as well as soot, dust, and other solid particles. This signal quality relates, for example, to a mass flow of the medium through the measuring channel leading to the sensor chip, and optionally to the reduction of a signal drift and the improvement of the signal-to-noise ratio. The signal drift relates to the deviation, for example of the mass flow of the medium, in the sense of changing the characteristic curve relationship between the mass flow actually occurring and the signal to be emitted within the scope of calibration during manufacture. For ascertaining the signal-to-noise ratio, the sensor signals which are output in a rapid time sequence are taken into account, whereas the characteristic curve drift or signal drift refers to a change in the mean value.
In customary hot film air mass meters of the described type, a sensor carrier having a sensor chip mounted thereon or inserted therein generally protrudes into the measuring channel. For example, the sensor chip may be glued into or onto the sensor carrier. The sensor carrier may form a unit with, for example, a base plate made of metal on which an electronics system and a control and evaluation circuit in the form of a printed circuit board may be glued. For example, the sensor carrier may be designed as a molded-on plastic part of an electronic module. The sensor chip and the control and evaluation circuit may be connected to one another via bond connections, for example. The electronic module produced in this way may, for example, be glued into a sensor housing, and the entire plug-in sensor may be closed by covers.
So that the hot film air mass meter is able to deliver an air mass signal which is as free of interference as possible, flow to the plug-in sensor and through the measuring channel in the plug-in sensor, and in particular over the measuring surface of the sensor chip, which is as uniform as possible, is important. It has been shown that the contour of the leading edge of the sensor carrier which protrudes into the measuring channel is of crucial importance for the signal quality of the sensor system. Thus, for example, it is proposed in published German patent application document DE 103 455 084 A1 that the leading edge of the sensor carrier to have a rounded design to improve the flow quality at the sensor carrier and at the sensor chip, and to avoid pulsing, nonstationary separations at the surface of the sensor chip.
Published German patent application document DE 10 2008 042 155 A1 discloses a sensor system for determining at least one parameter of a fluid medium, in particular an intake air mass of an internal combustion engine, flowing through a channel. The sensor system has at least one sensor chip situated in the channel for determining the parameter of the fluid medium. The sensor chip is accommodated in a sensor carrier which protrudes into the channel. The sensor carrier has a leading edge, situated transverse to the flow of the fluid medium, which has at least one vortex generator that is set up to form longitudinal vortices in the flowing fluid medium in the region of the sensor carrier. These longitudinal vortices require an improved intermixture between rapid fluid more remote from the wall and slower fluid, close to the wall and in danger of separation, in the region of the sensor carrier. This intermixture boosts an avoidance of separations. These required longitudinal vortices are generated by the vortex generator at the leading edge of the sensor carrier. This reduces the fluctuation in flow in the region of the sensor carrier, resulting in reduced signal noise and better reproducibility of the signal.
Despite the numerous advantages of the methods known from the related art for reducing the signal noise, these methods are still capable of improvement with regard to other functional aspects. Thus, due to the above-described deflection, only lighter particles such as dust, soot, or water and/or oil droplets, for example, pass into the bypass channel and into the measuring channel. Due to their mass inertia, heavier particles exit the hot film air mass meter through the main channel or bounce against the surrounding walls. The contamination of the sensor chip and its sensor area by the mentioned lighter particles, in particular oil and dust, results in an undesirable characteristic curve drift. Since the heat transfer in the region of the sensor area is determined primarily by the boundary layer flow on the sensor chip side of the sensor carrier, and also to a certain degree by the layer remote from the wall, in the sense of the fluid mechanics definition of a boundary layer, the flow over these flow regions must be as stable as possible from a topological standpoint. The contamination by particles may result in an altered topology over the long term, i.e., a change in the flow-defining structure at certain points, for example specks of dust, vortex foci, separation lines, and the like, or may result in a quantitative change in the flow variables.